Hot surfaces

Sulphur monochloride, S2CI2, m.p. —80 C, b.p. I38"C. Yellow liquid hydrolysed by water to SO2, HCl and S. Lower halides S CIj (jt up to 5) are formed S2CI2 plus H2 at a hot surface with freezing of products. S2CI2 is used in the rubber industry as a solvent for S. 

The second detergent function is to prevent formation of varnishes that come from polymerization of deposits on hot surfaces of the cylinder and the piston. Finally, by adsorption on metallic surfaces, these compounds have anti-corrosion effects. 

In Equation 7.1, n+/n is the ratio of the number of positive ions to the number of neutrals evaporated at the same time from a hot surface at temperature T (K), where k is the Boltzmann constant and A is another constant (often taken to be 0.5 see below). By inserting a value for k and adjusting Equation 7.1 to common units (electronvolts) and putting A = 0.5, the simpler Equation 7.2 is obtained. 

Consequendy, convective heat transfer determines the intensity of warming up and ignition. In addition, convective heat transfer also plays an important part in the overall dame-to-surface transmission. The reaction of gases is greatiy accelerated by contact with hot surfaces and, whereas the reaction away from the walls may proceed slowly, reaction at the surface proceeds much more rapidly. 

Free Hquid hydrocarbons and water flash vaporize if they contact hot surfaces. A rough estimate of the magnitude of such an event can be made if the free Hquid levels are known. Localized flashing of hydrocarbons and water continues, even iu the absence of free Hquids, whenever particles of waste are suddenly brought iu contact with hot surfaces or exposed to iatense radiation. 

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

Sheet Drying. At a water content of ca 1.2—1.9 parts of water per part of fiber, additional water removal by mechanical means is not feasible and evaporative drying must be employed. This is at best an efficient but cosdy process and often is the production botdeneck of papermaking. The dryer section most commonly consists of a series of steam-heated cylinders. Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely against the surface of the dryers by fabrics of carefuUy controUed permeabiHty to steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water evaporates. The water vapor is removed by way of elaborate air systems. Most dryer sections are covered with hoods for coUection and handling of the air, and heat recovery is practiced in cold climates. The final moisture content of the dry sheet usually is 4—10 wt %. 

Good housekeeping practices to prevent the accumulation of tantalum dust and a proper passivation procedure will prevent most tantalum fires. AH equipment used to handle the powder should be properly grounded and contact with hot surfaces or flames should be avoided. 

Most elements thermally vaporize as atoms but some, such as Sb, C, and Se, have a portion of their vapor as clusters of atoms. For these materials, special vaporization sources, called baffle sources, can be used to ensure that the depositing vapor is in the form of atoms by causing the material to be vaporized from multiple hot surfaces before it leaves the source. 

Vapors from Hquids can be put into the gas stream by bubbling the hydrogen or a carrier gas through the Hquid or by using a hot surface to vaporize the Hquid into the gas stream. Liquid precursors are generally metered onto a hot surface using a peristaltic pump and the gas handling system is kept hot to... 

Spray Pyrolysis. In spray pyrolysis, a chemical solution is sprayed on a hot surface where it is pyrolyzed (decomposed) to give thin films of either elements or, more commonly, compounds (22). Eor example, to deposit CdS, a solution of CdCl plus NH2CSNH2 (thiourea) is sprayed on a hot surface. To deposit Iu202, InCl is dissolved in a solvent and sprayed on a hot surface in air. Materials that can be deposited by spray pyrolysis include electrically conductive tin—oxide and indium/tin oxide (ITO), CdS, Cu—InSe2, and CdSe. Spray pyrolysis is an inexpensive deposition process and can be used on large-area substrates. 

Rapid cool-down by helium heat transfer is made possible at an interior of ca 100 Pa (0.1 atm). A convective fan transfers heat efficiently from the interior hot surfaces of the furnace to water-cooled base and wall parts. 

Hot surfaces and electric sparks are potential ignition sources for carbon disulfide. The ignition temperature depends on specific conditions, and values from 90 to 120°C in air have been reported (2,22). Data on carbon disulfide oxidation and combustion have been summarized (18). Oxidation products ate generally sulfur dioxide [7446-09-5] and carbon dioxide [124-58-9J ... 

Carbon tetrachloride [56-23-5] (tetrachloromethane), CCl, at ordinary temperature and pressure is a heavy, colorless Hquid with a characteristic nonirritant odor it is nonflammable. Carbon tetrachloride contains 92 wt % chlorine. When in contact with a flame or very hot surface, the vapor decomposes to give toxic products, such as phosgene. It is the most toxic of the chloromethanes and the most unstable upon thermal oxidation. The commercial product frequendy contains added stabilizers. Carbon tetrachloride is miscible with many common organic Hquids and is a powerhil solvent for asphalt, benzyl resin (polymerized benzyl chloride), bitumens, chlorinated mbber, ethylceUulose, fats, gums, rosin, and waxes. 

Ignition can also be produced by a heated surface. During the process of heat transfer from a hot surface to a flammable mixture, reactions are initiated as the temperature rises and the combination of additional heat transfer from the surface and heat release by chemical reactions can lead to ignition of the mixture. 

Insulation provides other functions in addition to energy conservation. A key role for insulation is safety. It protects personnel from bums and minimizes hot surfaces that could ignite inflammables. It also protects equipment, piping, and contents in event of fire. Thus materials such as mineral wool are sometimes used despite relatively poor thermal quaUties. 

External Conditions The principal external variables involved in any drying study are temperature, humidity, air flow, state of subdivision of the solid, agitation of the solid, method of supporting the solid, and contact between hot surfaces and wet solid. Alf these variables will not necessarily occur in one problem. 

When heat is transferred to a wet sohd by convection to hot surfaces and heat transfer by convection is negligible, the solids approach the boiling-point temperature rather than the wet-bulb temperature. This method of heat transfer is utilized in indirect diyers (see classification... 

Not apphcable Not apphcable Not applicable Different types are available for different requirements. Suitable for drying without contacting hot surfaces Not apphcable... 

Starch ana Other Flours Grinding of starch is not particularly difficult, but precautions must be taken against explosions starches must not come in contac t with hot surfaces, sparks, or flame when sus-... 

As a guide, because of convection that occurs from hot surfaces, ignition by a hot surface in open air should not be assumed unless the surface temperature is at least 200°C above the published minimum autoignition temperature (American Petroleum Institute, Ignition Risk of Hot Sui faces, API PSD 2216, Washington, 1980). 

Mechanically generated hot surfaces represent an ignition hazard if, irrespective of the MIT and the MIE, the. surface temperature is 1100°C or higher and the hot. surface area by itself is large enough (see Fig. 26-35). Higher surface temperatures and larger surfaces have a better incendivity lower temperatures and smaller surfaces have a poorer incendivity. 

FIG. 26-35 Equivalent energies of mechanically generated hot surfaces and lumps of smoldering material. 

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